Drilling riser

ABSTRACT

A marine riser, for use with floating drilling equipment, that has flexible joints attached to its upper and lower ends. The lower flexible joint connects the riser to a subsea wellhead. The upper joint links the riser to a vertically extensible joint which is in turn pivotally connected to the drilling vessel. A tensioning device aboard the vessel maintains the riser in tension, preventing it from buckling.

United States Patent [72} Inventor Danny R. Tidwell 3,177,954 4/1965Rand 175/7 Houston, Tex. 3,195,639 7/1965 Pollard et al... 175/7 X [21]Appl. No. 821,173 3,196,958 7/1965 Travers et a1. 175/7 [22] Filed May2,1969 3,313,345 4/1967 Fischer 166/.5 [45] Patented Aug. 24, 19713,353,851 11/1967 Vincent 166/.5 {73] Assign E850 Produm ResearchCompany Primary Examiner-Stephen J. Novosad Assistant Examiner-RichardE. Favreau 54] DRILLING RISER Attorneys-James A. Reilly, John B.Davidson, Lewis H.

7 Claims, 2 Drawing Fm Eatherton, James E. Gilchrist, James E. Reed andRobert L.

Graham [52] US. Cl 166/.5, 175]? 1 1 3 E21! 7/12 ansrnacr: A marineriser, for use with floating drilling [50] Field ofscarch 166/.5, .6;equipment, that has flexible join attached to i upper and 175/5, 7 lowerends. The lower flexible joint connects the riser to a subsea wellhead.The upper joint links the riser to a vertically ex- {56} Refemas Citedtensible joint which is in turn pivotally connected to the UNITED STATESPATENTS drilling vessel. A tensioning device aboard the vessel main-2,606,003 8/1952 McNeil] 175/7 tains the riser in tension, preventing itfrom buckling.

PATENTEU M824 m DANN Y R T/DWELL INVI'QN'IHR.

A T TORNE Y PATENTED AUB24I97I 3.601.187

SHEET 2 OF 2 im w u E 600 I n. m a I 0 I0 20 30 40 STRESS IN THOUSANDSOF POUNDS PER SQUARE INCH DANNY R TIDWE'LL lNVli/V'IUR.

BYwXIM ATTORNEY DRILLING RISER BACKGROUND OF THE INVENTION 1. Field ofthe Invention This invention applies to marine riser apparatus for usein drilling oil wells and similar boreholes from floating vessels.

2. Description of the Prior Art Most offshore drilling operationscarried out in deep water are conducted from floating vessels. Thismethod is particularly suited for such operations since the derrick,hoist, and associated drilling equipment are supported by the vessel,

eliminating the need for bottom-founded supports. The only substantialconnections between the floating vessel andthe ocean bottom are themooring lines and the riser pipe. The riser serves to guide the drillpipe into the well and conduct drilling fluid back to the vessel. Avertically extensible slip joint is placed in the upper end of the riserstring to prevent vertical motion of the vessel from damaging the riser.A tensioning device on the vessel applies tension to the riser toprevent it from buckling. The marine riser is generally regarded as alimiting element in floating drilling systems since the depth capabilityof present of present floating drilling vessels is to great extentgoverned by the stress level in the riser.

One problem with risers is structural failure caused by horizontalmovement of the vessel from the centerline of the wellbore. Wind, waves,and currents generally cause some vessel movement during drillingoperations even though the vessel is properly moored. It is thereforenecessary to increase the flexibility of the large diameter, relativelystiff riser to prevent riser failure. The use of one or more flexiblejoints placed intermediate the ends of the riser has been suggested as ameans of increasing riser flexibility. The approach generally taken,however, has been to attach the upper end of the riser to the drillingvessel by means of a pivotal connection and to join the'lower end of theriser to the blowout preventer stack with a flexible joint. Theseflexible risers may, however, fail in deep water because of overstressand fatigue. One reason for this is that the tension that must beapplied to theriser to prevent it from buckling increases rapidly withdepth. In addition, stress levels induced within the riser by waves,currents and the like also increase with depth, although at a somewhatlower rate than the tensile load. These increased stress levels coupledwith the cyclic variation in stresses caused by vessel heave lead toaccelerated-fatigue failure.

SUMMARY OF THE INVENTION The riser apparatus of the invention increasesthe depth capability of floating drilling equipment by reducing riserstresses. Studies have shown that peak stresses in conventional risersnormally occur in the upper section of the riser near the lower end ofthe slip joint. A major reason for this appears to be the large diameterof the slip joint relative to that of the riser. The combined effects ofthe relatively large force imparted to the large diameter slip joint bywave action and the relative stiffness of the slip joint compared tothat of the riser create a high level of stress in the riser justbeneath the slip joint. It has been found, however, that placing aflexible joint in the riser string in close proximity to the lower endof the slip joint substantially reduces the maximum stress. Such a jointextends the maximum operating depth of any given riser configuration or,alternatively, increases riser operating life at any particularoperating depth.

The riser apparatus of the invention includes a riser pipe, a flexiblejoint attached to the lower end of the riser pipe, means for connectingthis flexible joint to wellhead equipment fixed to a subsea bottom, aflexible joint attachedto the upper end of the riser, a slip jointattached to the upper end of this flexible joint, means for applying anaxial tensile load to the riser, and means for pivotally connecting theupper end of the riser pipe to the drilling vessel. The preferredflexible joint is a pressure balanced ball joint, although other jointsthat remain flexible under high axial loadings may be used.

The apparatus of the invention substantially increases the life ofrisers in floating drilling operations, in both deep and shallow water.In addition, the new riser system has made it possible to drill safelywith fluid returns in water depths ex-. ceeding 1,250 feet. 'As aresult, the system has wide application and is particularly useful indeep water areas where the limitations of conventional riser mayotherwise preclude effective drilling operations.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERREDEMBODIMENT FIG. 1 shows riser apparatus built according to, theinvention for use with a floating drilling vessel. A drilling vessel llis shown floating on a body of water 13. A marine riser l5 which hasflexible joints attached to its upper and'lower ends, 17 and 19respectively, extends downwardly beneath the vessel. The upper joint 17links the riser with a vertically extensible slip joint 21 which in turnis pivotally connected to the drillingvessel by cable 23 and tensioners25 acting through cable 41. These tensioners maintain'the riser stringin tension to prevent it from buckling. I V

The riser pipe 15 is normally composed of a series of short joints.These joints may be threaded together or joinedin some other fashion.The riser guides drill pipe into subsea well 27 and conducts drillingfluid returns back from the subsea well to the vessel. It will thereforenormally-be a relatively large diameter conduit through which the drillpipe and the drill bit may pass. It should have sufficient burstresistance "to withstand the differential pressures generated by the useof drilling fluids with densities of 18 pounds per gallon or more on theinside and sea water with a density of about 8.5 pounds per gallon onthe outside. The riser should also have the strength to withstand thetensile stresses generated in response to the forces applied to maintainit in a substantially vertical position. i

The lower ball joint 19 links the riser to a blowout preventer 29, whichis attached in turn to a subsea wellhead, 31. The lower ball jointprevents the transfer of moments from" the riser to the wellhead andblowout preventer stack and allows".

the riser to flex when the vessel is moved away from the cen terline ofthe wellbore. While this flexible joint should be located in the riserpipe string close to the blowout preventer stack, it need not be joinedto the stack. Large diameter ball joints have been found to makeparticularly effective flexible joints for this purpose, but anyflexible joint that can withstand both tensile and compressive loadswould do. In deep water operations it may be necessary to balance theaxial forces that act on the ball joint to assure' its flexibility. Apressure balanced ball joint may be used to accomplish this purpose. Onejoint that has been found particularly suitable is the type CR-l balljoint manufactured by Regan Forge and Engineersion to the riser stringto prevent it from buckling. Tensioner 25 develops the tensile force,and acting through cable 41' which passes over pulley 43 and is attachedto. outer barrel clamp 39, imparts a tensile load to the outer barrel ofthe slip joint. This in turn is transmitted to the whole riser pipestring.

The tensioner may be hydraulically actuated and may reel the cable inand out in response to vertical motion of the vessel, thus maintaining asubstantially constant tensile load on the riser. Generally, time delaysare experienced with such tensioners and result in a variation in thetensile load applied to the riser on the order of :1 5 percent.

The inner barrel of the slip joint is pivotally connected to thedrilling vessel by means of a gimbal or a system of cables 23 as shown.Since the riser string is pivotally connected at its upper end to thevessel and flexibly connected to the blowout preventer stack at itslower end, it is free to flex when the drilling vessel is displaced fromthe centerline of the wellbore, preventing failure of the riser. v i

As indicated above, it has been found that peak stresses normally occurat the upper end of the riser pipe. Curve A of F IG. 2 exemplifies thevariation of stress with depth for a conventional riser string having noupper ball joint. This example is based on a typical semisubmersibledrilling vessel drilling in water 400 feet deep. The riser has a16.0-inch diameter, contains a 10.0 pound per gallon drilling fluid andis subjected to a 220,000-pound-tensile load by the tensioners. Aone-knot current is assumed to act on the upper percent of the riser,and a 0.5-knot current is assumed to act on the lower 90 percent. Waveheight is 30 feet and the vessel is positioned directly above thecenterline of the wellbore. Curve A of FIG. 2 shows that the maximumriser stress reaches a level of some 39,000 p.s.i. at 95 feet, which isjust below the slip joint.

, lt has been found that peak riser stress can be reduced by placing aflexible joint in the riser string in close proximity to the lower endof the slip joint. The flexible joint increases riser flexibility at thebase of the slip joint, substantially preventing the transfer of momentsfrom the slip joint to the riser pipe. Large diameter ball joints havebeen found to make particularly effective flexible joints but anyflexible joint that can withstand both tensile and compressive loadswill do. It has been found desirable to use a pressure balanced balljoint in string of Curve A. All other conditions remain the same. The

maximum riser stress shown in Curve B again occurs near the,

lower end of the slip joint, but the addition of the flexible joint hasreduced the peak stress from 39,000 to 15,000 p.s.i. This reduction inpeak stress substantially extends the depth capability of such a riserstring. This is illustrated by Curve C of F K}. 2 which shows the stressdistribution of a riser string like that in Curve A but with water depthincreased from 400 to 1,200 feet. All other conditions are assumed toremain the same. Riser stress in Curve C again peaks near the lower endof the slip joint, but the maximum stress is only l7,.000 p.s.i. This iswell'below the peak stress in the conventional riser in 400 feet ofwater (Curve A), despite the fact that the riser has been extended anadditional 800 feet.

The stress reduction achieved by placing a flexible joint in the upperend of the riser string will also increase the fatigue life of theriser. Fatigue life is a function 'of both the mean maximum riser stressand the magnitude'of the cyclical stress variation from that meanstress. Fatigue life of the riser can be extended by reducing either themean maximum riser stress or the magnitude of the cyclical variations.Cyclical stress variations are caused primarily by vessel motion and areinfluenced by the sea state to which the vessel is exposed. Themagnitude of the cyclical stress variations is not substantiallyafi'ected by changes in the configuration of the drilling riser but themean maximum stress when an upper flexible joint is added will besubstantially lower than that which would be experienced with aconventional riser. The mean maximum stress for Curve 8 will be seen tobe only 34 percent of that for a conventional riser. The riser of theinvention will therefore have a substantially longer fatigue life than aconventional riser.

What is claimed is: 1. Apparatus for use In an a. a substantiallyvertical riser; b. an extensible joint attached to the upper end of theriser; c. a flexible joint in said riser positioned below and in closeproximity to the extensible joint; d. means for flexibly connecting thelower end of the riser to a subsea wellhead; e. means for pivotallyconnecting the extensible joint to a floating vessel; and f. means forapplying tension to the riser; 2; Apparatus as defined by claim 1wherein the flexible join is a ball joint.

3. Apparatus as defined in claim 2 wherein the means for flexiblyconnecting the lower end of the riser to the subsea wellhead includes aball joint.

4. Apparatus for use in a well operation conducted from a floatingvessel comprising:

a. a substantially vertical riser; b. a first flexible joint attached tothe lower end of the riser; c. means for connecting the first flexiblejoint to a subsea wellhead;

ofi'shore well operation comprisd. a second flexible joint in said risernear the upper end thereof;

e. a slip joint attached to the upper end of the riser;

f. means for pivotally connecting the slip joint to the vessel;

g. means on the vessel for generating a tensile force; and

h. means for transmitting said tensile force to said riser.

5, Apparatus as defined by claim 4 wherein the second flexible joint isa ball joint.

6. Apparatus as defined by claim 5 wherein the first flexible joint is aball joint.

7. Marine riser apparatus comprising:

a. a substantially vertical riser;

b. a'first ball joint attached to the lower end of the riser;

c. means for connecting the ball joint to a subsea wellhead;

d. a second ball joint attached to the upper end of the riser;

e. a slip joint attached to the upper end of the second ball joint;

f. means for pivotally connecting the slip joint to a floating vessel;and

g. means for applying tension to the riser.

1. Apparatus for use in an offshore well operation comprising: a. asubstantially vertical riser; b. an extensible joint attached to theupper end of the riser; c. a flexible joint in said riser positionedbelow and in close proximity to the extensible joint; d. means forflexibly connecting the lower end of the riser to a subsea wellhead; e.means for pivotally connecting the extensible joint to a floatingvessel; and f. means for applying tension to the riser.
 2. Apparatus asdefined by claim 1 wherein the flexible joint is a ball joint. 3.Apparatus as defined in claim 2 wherein the means for flexiblyconnecting the lower end of the riser to the subsea wellhead includes aball joint.
 4. Apparatus for use in a well operation conducted from afloating vessel comprising: a. a substantially vertical riser; b. afirst flexible joint attached to the lower end of the riser; c. meansfor connecting the first flexible joint to a subsea wellhead; d. asecond flexible joint in said riser near the upper end thereof; e. aslip joint attached to the upper end of the riser; f. means forpivotally connecting the slip joint to the vessel; g. means on thevessel for generating a tensile force; and h. means for transmittingsaid tensile force to said riser.
 5. Apparatus as defined by claim 4wherein the second flexible joint is a ball joint.
 6. Apparatus asdefined by claim 5 wherein the first flexible joint is a ball joint. 7.Marine riser apparatus comprising: a. a substantially vertical riser; b.a first ball joint attached to the lower end of the riser; c. means forconnecting the ball joint to a subsea wellhead; d. a second ball jointattached to the upper end of the riser; e. a slip joint attached to theupper end of the second ball joint; f. means for pivotally connectingthe slip joint to a floating vessel; and g. means for applying tensionto the riser.